Electronic circuitry in the region of the interconnection between a die and a flexible circuit on print head assemblies is prone to corrosion. The rate of corrosion is accelerated by the presence of ink, which accumulates in this region following the wiping of the nozzle plate to ensure good print quality. The interconnect region on print heads is commonly protected by the application of a polymeric protective layer, which is also known as encapsulant layer (see, U.S. Patent Application 2003/0017341 or U.S. Pat. No. 6,439,698). While these materials show good resistance to ink permeation, with inks becoming increasingly corrosive, problems are experienced with penetration of ink at the encapsulant/print head interface, which can lead to device failure.
Inorganic materials such as silica, alumina and tantala are known to produce efficient protective layers. However, these materials cannot be selectively deposited onto the interconnect region at this stage (connection of the print head to the flexible circuit) of the print head assembly process.
For other parts of ink jet print heads, the use of layers comprising cured polymer material is known. The use of photoimageable layers containing epoxy functional and methacryloxy functional silanes to produce channel structures, jet plates, reservoirs, ink filters and passivation layers of a print head is disclosed in U.S. Pat. No. 6,312,085. Similar materials are also used in U.S. Pat. No. 6,283,578 for the preparation of hydrophobic coating layers to control the wetting of ink on the surface of ink jet nozzle plates). The coating materials described in U.S. Pat. Nos. 6,283,578 and 6,312,085 require a long thermal curing step at elevated temperature in order to effectively cross-link the silanol groups which are present in the materials after curing by ultraviolet (UV) light. This long processing time renders these materials unsuitable for applications in which automated manufacturing processes, such as reel-to-reel processing, are employed that do not allow extended waiting times.
Furthermore, U.S. Pat. No. 5,910,372 discloses the use of a formulation based on a mixture of silanes with different functional groups for the preparation of hydrophobic coating layers on ink jet nozzle plates. Silanes containing amino groups are incorporated to improve adhesion to the nozzle plate, which is made of polyimide, while perfluoroalkyl substituted silanes are included to provide the required level of hydrophobicity. This patent advances an earlier technology described in U.S. Pat. No. 5,121,134 in which the materials were applied separately in two coating steps. These materials also require a long thermal curing step at elevated temperature which makes them unsuitable for use in manufacturing processes such as reel-to-reel processing.
Problems are experienced with ink penetration to the interconnection region being enabled by delamination of the protective layer from the various surfaces present on the print head. Delamination allows ink a ready path to the interconnect region by diffusion at the interface between the protective layer and the print head. These problems are becoming more difficult to overcome as ink formulations become more aggressive, while at the same time increased levels of product reliability are expected.
Accordingly, there remains the need for a coating material that shows low ink uptake, thus limiting the penetration of ink through this protective coating. Such a coating should also show good adhesion properties to the various surfaces present on a print head and its adhesion should not be degraded by exposure to ink.